Sets keyboard focus to popup.

<div class="definition">
    focus()
</div>

<h4>Description</h4>

This is internal function that is called when popup needs to get focus from keyboard. Because the content behind popup is locked, there should be no need to call
this function manually.
<div style="height: 10px"></div>

To set keyboard focus to the popup:
<textarea class="javascript">
w2popup.focus();
</textarea>